>> reporter: tracy lippincott, who works in a san francisco bar, is taking a college course in her apartment, online, on how to reason and argue. the teacher is walter sinnott- armstrong, professor of ethics at duke university in north carolina, and the class is free. >> so how do you learn the technique? the answer is very simple. you practice, and then you practice again, and then you practice and practice and practice and practice. this class has these really short little lectures, which is great because you can kind of watch one, and then think about it and react, and then you don't have to watch another whole hour like you would in class. >> reporter: "think again" is a class presented by a one-year- old for-profit startup called coursera, currently the nation's largest provider of free online courses. 170,000 students from around the world have signed up for it. the classes are called moocs, or massive open online courses, and they may be revolutionizing higher education.